More efficient receipt cleaning

When a validator applies a block, he needs to remove the applied operations from his mempool. Currently the mempool is a standard OCaml map, so removing the applied operations is O(n*log m) where n is the operations in a block and m is the size of the mempool.

We should switch to a more efficient data structure.

No file or test is named. Start by locating the validator block-application path and the mempool implementation, then inspect how applied operations are removed from the OCaml map; done means a more efficient data structure reduces receipt-cleaning work from O(n*log m).

Written by the indexing model from the issue text.